Tuesday, November 01, 2016

ALMA image of the L1448 IRS3B system, with two young stars at the center and a third distant from them. Spiral structure in the dusty disk surrounding them indicates instability in the disk, astronomers said. Credit: Bill Saxton, ALMA (ESO/NAOJ/NRAO), NRAO/AUI/NSF.

Combined ALMA and VLA image of L1448 IRS3B system.

Credit: Bill Saxton, ALMA (ESO/NAOJ/NRAO), NRAO/AUI/NSF.

Artist's conception of how the triple-star system develops.

Left, disk of material fragments into separate protostars.

Right, the resulting stellar system.

Credit: Bill Saxton, NRAO/AUI/NSF.

For the first time, astronomers have seen a dusty disk of material
around a young star fragmenting into a multiple-star system. Scientists
had suspected such a process, caused by gravitational instability, was
at work, but new observations with the Atacama Large
Millimeter/submillimeter Array (ALMA) and the Karl G. Jansky Very Large
Array (VLA) revealed the process in action.

"This new work
directly supports the conclusion that there are two mechanisms that
produce multiple star systems -- fragmentation of circumstellar disks,
such as we see here, and fragmentation of the larger cloud of gas and
dust from which young stars are formed," said John Tobin, of the
University of Oklahoma and Leiden Observatory in the Netherlands.

Stars
form in giant clouds of gas and dust, when the tenuous material in the
clouds collapses gravitationally into denser cores that begin to draw
additional material inward. The infalling material forms a rotating disk
around the young star. Eventually, the young star gathers enough mass
to create the temperatures and pressures at its center that will trigger
thermonuclear reactions.

Previous studies had indicated that
multiple star systems tend to have companion stars either relatively
close, within about 500 times the Earth-Sun distance, or significantly
farther apart, more than 1,000 times that distance. Astronomers
concluded that the differences in distance result from different
formation mechanisms. The more widely-separated systems, they said, are
formed when the larger cloud fragments through turbulence, and recent
observations have supported that idea.

The closer systems were
thought to result from fragmentation of the smaller disk surrounding a
young protostar, but that conclusion was based principally on the
relative proximity of the companion stars.

"Now, we've seen this disk fragmentation at work," Tobin said.

Tobin,
Kaitlin Kratter of the University of Arizona, and their colleagues used
ALMA and the VLA to study a young triple-star system called L1448
IRS3B, located in a cloud of gas in the constellation Perseus, some 750
light-years from Earth. The most central of the young stars is separated
from the other two by 61 and 183 times the Earth-Sun distance. All
three are surrounded by a disk of material that ALMA revealed to have
spiral structure, a feature that, the astronomers said, indicates
instability in the disk.

"This whole system probably is less than
150,000 years old." Kratter said. "Our analysis indicates that the disk
is unstable, and the most widely separated of the three protostars may
have formed only in the past 10,000 to 20,000 years," she added.

The
L1448 IRS3B system, the astronomers conclude, provides direct
observational evidence that fragmentation in the disk can produce young
multiple-star systems very early in their development.

"We now
expect to find other examples of this process and hope to learn just how
much it contributes to the population of multiple stars," Tobin said.

The scientists presented their findings in the October 27 edition of the journal Nature.

ALMA
is a partnership of ESO (representing its member states), NSF (USA) and
NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and
KASI (Republic of South Korea), in cooperation with the Republic of
Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ.

The
National Radio Astronomy Observatory is a facility of the National
Science Foundation, operated under cooperative agreement by Associated
Universities, Inc.